The present invention generally relates to information signal recording media and in particular to a disk-shaped information recording medium on which an information signal is recorded by means of an energy beam such as a laser beam (optical beam) or an electron beam.
Optical information recording medium such as an optical disk or a magneto-optical disk hereinafter referred to as a disk is recorded with an information signal such as a video signal or an audio signal modulating an optical beam which moves along a spiral or concentric guide groove provided on the surface of the disk. At the time of reproduction, the guide groove is irradiated by an optical beam and the information signal is reproduced by processing the optical beam reflected back from the guide groove. For this purpose, tracking of the the optical beam must be controlled such that the optical beam traces the guide groove properly. Such a tracking of the optical beam is achieved by means of a known servo control system which controls the optical beam on the basis of the optical beam reflected back from the guide groove. The same tracking principle is known to be applicable to recording the information along a preformed guide track. Thus, the guide groove is used not only for storage of information signals but also for maintaining a proper tracking of the optical beam at the time of recording and reproducing of the information signal on and from the disk. The guide groove is usually a spiral-shaped or concentric continuous groove but may be a series of intermittent pits as in the case of a reproducing only type optical disk such as a so-called Compact Disk.
The information recording medium having the guide groove or pits as aforementioned is manufactured by impressing a pattern on a metal stamper which is an inversion of the pattern of the groove or pits to be formed on the surface of the disk. The disk may be formed by injection molding or compression molding of a thermoplastic resin using the metal stamper as the mold. The manufacture of the disk by the injection molding or compression molding has a high productivity and is suited for automatic production. On the other hand, the disk thus produced has a problem in that the impression of the guide groove or pits by the stamper is not satisfactorily precise. Further, the disk tends to show birefringence, and the disk is deformed by the moisture in the air. Thus, these problems of the conventional plastic disk will create difficulties at the time of recording and reproducing.
In order to eliminate these problems, use of silica glass for the substrate of the disk is proposed. The use of silica glass as the substrate of the disk is advantageous in that the disk thus produced has a small thermal expansion and shows virtually no absorption of moisture. Further such a disk has negligible birefringence. In order to provide the guide groove or pits on the surface of such silica glass disk, a layer of UV-cure resin which is a resin cured by ultraviolet radiation is deposited on the surface of the glass. In detail, a layer of UV-cure resin is first applied to the surface of the stamper in an uncured state and the resin is covered by the silica glass disk so as to be sandwitched between them. Next, an ultraviolet light is irradiated on to the resin through the glass disk and the resin is cured. The stamper is then removed and a two layered disk comprising a glass substrate and a layer of the cured resin carrying the pattern of groove or pits thereon is obtained. The disk thus produced is superior as compared with the plastic disk of thermoplastic resin in that a resin having a low viscosity at room temperature can be used and the groove or pits on the stamper is transferred to the plastic layer more accurately as compared to the case of the conventional disk molded from the usual thermoplastic resin. However, this procedure involves delicate steps of sandwitching uncured resin layer as well as of the separation of the stamper from the cured resin layer which pauses difficulties in automatization of its manufacture.
The disk produced by molding of the thermoplastic resin or by application of the UV-cure resin on the silica glass substrate is further deposited with a reflection layer by vacuum vapor deposition or by sputtering. During such procedure, there is a problem that water is released from the cured resin or molded plastic due to the heating and the structure and property of the reflection layer become deteriorated.
In order to eliminate this problem, provision of the guide groove or pits directly etched on the surface of the silica glass substrate is proposed in the U.S. Pat. No. 4,655,876 as well as in the Japanese Laid-open Patent Application No. 26951/1986 in which the respective assignee and the applicant are same as the assignee of the present application. According to the procedure proposed by the aforementioned patent and patent application, a layer of photoresist is applied on a polished surface of a silica glass substrate. Then, the pattern of guide groove or pit is written on this photoresist by means of a focused laser beam. Then, after a development of the laser exposed photoresist, the substrate is subjected to a dry etching such as a plasma etching using a plasma gas such as CF.sub.4. The plasma gas selectively reacts with the silica of the glass and the silica material at the portion of the disk not covered with the photoresist is removed by the reaction. On the other hand, the portion of the silica masked by the photoresist is not subjected to the reaction. The reaction is continued until an intended groove depth is reached. After the groove reaches the intended depth, the plasma gas is changed to a gas containing oxygen (O.sub.2) and the remaining photoresist is removed by reaction with the oxygen.
However, the disk thus produced shows an unsatisfactory signal-to-noise ratio when the recording and reproduction is made after deposition of recording layer and protection layer on the disk. More specifically, error in the reproduced signal as well as the tracking error of the optical beam were found to be excessive for a satisfactory recording and reproducing operation of the disk. The reason for this was studied by electron microscopic observation of the disk and it was discovered that deterioration in the S/N ratio is caused by the irregularity or roughness at bottom of the groove. Such irregularity produces an unstable reflection of the optical beam. When the surface roughness exceeds about 100 .ANG., the reflected optical beam becomes too unstable for satisfactory operation of the recording and reproducing system and the proper reproduction of the information signal or proper tracking of the optical beam is lost. At present, it is impossible to etch silica glass without causing irregularity at the bottom of the groove. Thus, it is not possible to obtain disk to provide satisfactory recording and reproducing results.